Serotonin plays critical roles as a neurotransmitter, neuromodulator and hormone. Therapeutics for many psychiatric disorders, including major depression, anxiety disorders and eating disorders, target serotonin signaling pathways in the brain. Understanding molecular mechanisms that regulate serotonin signaling is therefore essential both for understanding the causes of many types of neurological and psychiatric illness and for developing new therapeutics. The roundworm C. elegans is a powerful model for genetic and molecular studies of nervous system function. A synapse between serotonergic motor neurons and muscles of the reproductive system drives the reproductive behavior of the C. elegans hermaphrodite, egg laying. Through genetic studies of egg-laying behavior, we have discovered that neuropeptides modulate serotonin signaling in the C. elegans reproductive system by directly inhibiting serotonergic motor neurons. Some of these modulatory neuropeptides are provided by a pair of sensory neurons, the BAG neurons, which are stimulated by environmental carbon dioxide. The aim of this proposal is to determine physiological and molecular mechanisms required for the activation of peptidergic sensory neurons by carbon dioxide, and mechanisms by which neuropeptides inhibit the serotonergic neurons that they target.